[FLINK-31337][python] Fix python keyed broadcast batch operator input not sorted

[Vancior]

What is the purpose of the change

This PR fixes elements are not sorted under the same key in Python keyed broadcast operator under batch mode.

Verifying this change

This change is already covered by existing tests, test_keyed_co_broadcast_* in test_data_stream.py, which accidently works under general configuration parallelism=2 before this fix.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable)

[flinkbot]

CI report:

• 1928295 Azure: CANCELED
• 0fa77b7 Azure: PENDING

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